With high integration and large capacity of a Large Scale Integration (LSI), a circuit dimension required for a semiconductor element becomes increasingly narrow.
Using an original image pattern (that is, a mask or a reticle, hereinafter collectively referred to as a mask), a reduced-projection exposure apparatus called a stepper or a scanner exposes and transfers the pattern on a wafer to form a circuit, thereby producing the semiconductor element.
It is necessary to improve a production yield for costly LSI production. On the other hand, there is a demand for pattern formation having a line width of some dozen nanometers in a contemporary device. At this point, a defect of the mask pattern can be cited as a large factor that degrades the production yield. The finer the dimensions of an LSI pattern formed on a semiconductor wafer becomes, the finer the defect of the mask pattern becomes.
As fluctuations of various process conditions are absorbed by enhancing dimensional accuracy of the mask, it is necessary to detect the defect of the extremely small pattern in a mask inspection. Therefore, high accuracy is required for an apparatus that inspects patterns of a mask.
In a mask inspection apparatus, light output from a light source is emitted onto a mask through an optical system. The mask is mounted on a stage, and the emitted light scans the mask while the stage moves. The light transmits or is reflected with respect to the mask, and passes through a lens to image on a sensor. Then, the defect inspection is performed based on optical images acquired by the sensor.
Examples of mask inspection methods using a mask inspection apparatus include a die-to-die comparison method and a die-to-database comparison method. In the die-to-die comparison method, an optical image is compared with another optical image of the same pattern as the optical image at a different location. On the other hand, in the die-to-database comparison method, a reference image generated from design data (CAD data) used in mask production and an optical image of the actual pattern on the mask are compared to each other.
In order to generate an optical image, a charge storage type time delay integration (TDI) sensor and a sensor amplifier that amplifies an output of the TDI sensor are used. In a case where an inspection is performed by transmitted light, for example, a halftone type phase shift mask can obtain a contrast of a light shielding film and a glass substrate to some degree. Therefore, as in a chrome mask, there is adopted a method of determining a defect by recognizing a mask pattern using a light intensity signal of a sensor image that is light-received by a detection optical system.
Depending on the shape of the defect, the contrast may be easily obtained when reflected light of a mask surface is used. There is also an inspection method using a reflection inspection optical system for the purpose of a foreign particle inspection function or the like. Also, there is disclosed a mask inspection method capable of performing a defect inspection with high detection sensitivity by easily correcting a focus deviation of transmitted illumination light caused by variation in a mask thickness (see JP 2008-249921 A).
In the comparison between the reference image and the optical image, it is known to perform the calibration of the gain and the offset for sensor amplifier output adjustment, using the calibration pattern. In this calibration, there is also a case where miniaturization of a pattern etched on a mask is progressed and a black region or a white region of a sufficiently wide area does not exist in a mask pattern itself. There is also a case where it is difficult to provide the calibration pattern in the inspection target mask due to the expansion of the area occupied by the product pattern.
In a case where it is difficult to provide the calibration pattern in the inspection target mask, a calibration mask in which a black region and a white region are formed on a film of the same type as that of the inspection target mask and which calibrates an offset and a gain (hereinafter, simply referred to as a calibration mask) is used.
In a case where the calibration is performed using the calibration mask, after the calibration is performed, the mask defect inspection is performed on the inspection target mask. On the other hand, there is also disclosed a mask inspection method in which the offset gain of the sensor amplifier can be calibrated using the inspection target mask even when a black region and a white region of a sufficient size as compared with an imaging area of a TDI sensor do not exist in the inspection target mask (see JP 2009-300426 A).
However, for example, in a case where a mask with a pellicle is inspected, since a region necessary for light quantity calibration does not exist in the pellicle, the light quantity calibration is performed using a calibration mask with a pellicle separately from the mask to be inspected.
In the calibration using the calibration mask, for example, filter conditions of the optical system as well as gain and offset values to be set to the TDI sensor are stored in a file. The stored file is read and the filter conditions of the optical system as well as the gain and offset values of the TDI sensor are set.
In the calibration using the calibration mask, apparatus states, such as the light quantity of the light source, are changed at the time of the actual inspection. Thus, an appropriate calibration operation is not performed and a false defect induction may be caused.
In the mask inspection apparatus and the mask inspection method that perform the inspection using the calibration mask, normalization of the light quantity calibration sensor performing the calibration of the light quantity change is executed when the light quantity calibration and the file reading light quantity calibration are performed. That is, the relationship between the light quantity and the TDI sensor output is made constant and the determination of the gain is also performed. The normalization of the light quantity calibration sensor is performed whenever the light quantity calibration is performed.
In such a calibration, in a case where the light quantity at the time when the light quantity calibration is performed using the calibration mask is different from the light quantity at the time when the inspection target mask is inspected, a gray scale value of the TDI sensor image is different from a specified value, which results in a false defect.
The present invention has been made in view of the issues described above. That is, an object of the present invention is to provide a mask inspection apparatus and a mask inspection method, which are capable of suppressing a gray scale value of a sensor image from being different from a specified value in a case where a light quantity at the time when a light quantity calibration is performed using a calibration mask is different from a light quantity at the time when an inspection target mask is inspected.